Gas-liquid Partition Chromatography D. H. LICHTENFELS, S. A. FLECK, G u l f Research
&
and
F. H. BUROW
Development Co., Pittsburgh, Pa.
The technique of gas-liquid partition chromatography was investigated as a method for analyzing complex organic mixtures. This technique was found to be very rapid and versatile for the qualitative and quantitative analysis of complex hydrocarbon mixtures in the Cs to CSrange. The separations obtained by this method in a few minutes are similar to those obtained on precision distillation columns requiring several hours of operation. The separated components of a mixture are detected with a thermal conductivity cell as they emerge from the column. A complete description of the apparatus and the analytical results for several typical samples are presented to show the accuracy and versatility of this technique. This technique is now in use, both as an independent analytical tool and as a powerful adjunct to molecular spectroscopic procedures.
D
URIXG recent years, numerous papers have appeared in the literature describing the scope and application of chromatographic methods of analysis. Recently chromatographic methods have been developed for the separation and analysis of gases and volatile liquids using the adsorption method (2,3,10-13) and the gas-liquid partition method (1, S-10, 13, 14). I n the adsorption method, the mixture to be separated is adsorbed in a narrow band a t the one end of a column filled with an adsorbent such as activated charcoal. Different authors have described techniques in which the sample is eluted from the adsorbent by a carrier gas. Recently a commercial apparatus (2) was made available in which the adsorbed material is displaced from the adsorbent by passage of a more strongly adsorbed vapor through the column.
Figure 1.
as dioctyl phthalate. The column is prepared in a manner einiilar to that described by James and Martin (7’). When a lower boiling mixture is charged to the end of the column, the individual components will partition between a gas phase in the pore space and a liquid phase absorbed in the high boiling organic coating. The column is then eluted with an inert gas which causes the components to move forward with individual velocities, which are less than that of the carrier gas. The velocity with which a particular component moves is dependent upon its partition coefficient. Since the partition coefficient varies for different compounds, a separation into zones results within the column. The separated components are detected with a thermal conductivity cell (Gow-Mac Instrument Co., RIadison, K,J . ) as they emerge from the column. Gas-liquid chromatographic techniques have been shown to be a poiverful and versatile tool for the analysis of organic compounds. For those interested in rapid and accurate means for analyzing hydrocarbons, this method has several advantagesnamely, good separation is obtained readily: small samples are used, since analyses are run on only a few milligrams of sample; azeotrope formation causes no trouble; and only simple apparatus, easy to operate, is required. The separations obtained are similar to those obtained on precision distillation columns. Hence the method can be used as a poiverful adjunct to molecular spectroscopic techniques. The gas-liquid partition columns have two main advantages over the liquid-liquid partition columns. The lo^ viscosity of the gas phase allows relatively longer columns to be used with a corresponding gain in efficiency. Also the methods for detecting a change in composition of a gas stream are generally simpler than those for a liquid stream. APPARATUS
The schematic diagram of the apparatus is shown in Figure 1. I n operation, a stream of carrier gas is continually passed through the system in the direction indicated by the arrows. The carrier gas is drawn from a cylinder through a series of reducing valves and finally a needle valve in order to maintain a constant flow of gas. The flow of carrier gas is indicated b)’ a rotameter and measured with a wet test meter. The liquid nitrogen trap is used when fractions are collected for further analysis by the mass spectrometer. The electrical bridge of the thermal conductivity cell is balanced with carrier gas passing through both the reference channel and the sample channel. The top of the column is equipped with a rubber serum bottle cap for injecting the sample to be analyzed by means of a hypodermic syringe. During elution of a component from the hvdrocarbon mixture, the sample channel of the thermal conductivity cell is exposed to a mixture of carrier gas plus the component leaving the column. This results
Schematic diagram of apparatus
The techniques of gas-liquid partition chromatography are similar in many ways to the liquid-liquid chromatographic columns except that the mobile phase is a gas instead of a liquid Briefly the basis of this method is as follows: 8
104
9 sample of the mixture to be analyzed is injected into the end
8
96
,
ea
60
72
of a narron- column packed with an inert granular material (Celite 545, Johns RIanville Corp., Pittsburgh, Pa.) on which has been deposited a coating of a very high boiling organic liquid such
Figure 2.
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64 TIYE
IN
1
LO
1
,
24
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Chromatogram of synthetic saturated hydrocarbons
mixture of
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V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5 in an unbalanced voltage in the thermal conductivity cell bridge. This unbalanced voltage is fed to a recording potentiometer which automatically plots detector response versus time. An example of this type of plot is illustrated in Figure 2, which is the chromatogram of a synthetic mixture of saturated hydrocarbons. This sample was run in a column 10 feet long with an inside diameter of 4.5 mm. a t a temperature of 65' C. and a gas flow rate of 29 ml. per minute, using dioctyl phthalate as the liquid phase. This curve show a continuous plot of the signal from the thermal conductivity detector sampling the gases leaving the column versus time of elution. The qualitative identification of each hydrocarbon is based on its retention volume. Because a constant flow rate of carrier gas 7yas used for each run, it is more convenient to plot retention time instead of retention volume. The concentration of each component is determined from the areas under the peaks, as the response of the thermal conductivity cell for the hydrocarbons in this molecular neight range is essentially the same. I t is seen from this chromatogram that in nuxtures containing components n.ith a wide range of boiling points, the higher boiling components tend to spread themselves out to 'give a long band of low concentration. For qualitative n.ori;, this difficulty can be easily overcome by increasing the column temperature during a run. For quantitative xork, this procedure is undesirable, as it requires exact reproduction of temperature rise and flow rate, as well as calibrated peak areas for components a t different temperatures.
ured retention time for each compound is the number of minutes between the air peak resulting from the injection of the component and the detection of mavimum concentration of that component in the effluent gas. The retention time of a compound depends on the boiling point and relative solubility between the compound and the organic coating in which it is absorbed in the column 5
N-PEL
4 c
1:
5 : 3 2
0 n
SOPENTANE
u)
W
7
(z
a
0
+ 2
0
w
c W
n ISOBUTANE
I 2-METHYL -2-BUTENE 5
1
2-METHYL-2-BUTENE
1 30
25
20 15 -MINUTES
IO
5
0
Figure 4. Chromatogram of typical Cb and lighter hydrocarbon sample
0
35
30
25
20
35
30
25
20
15
-MINUTES
Figure 3. Special separations using different liquid coatings
Table I gives the order of elution for several of the saturated hydrocarbons in the light gasoline range. These data apply to a column 10 feet long with an inside diameter of 4.5 mm. operated a t 65" C. and a carrier gas (hydrogen) flow rate of 28 ml. per minute, using dioctyl phthalate as the liquid phase. The meas-
Table I. Component Isopentane n-Pentane 2,2-Dimethylbutane 2,3-Dirnethylbutane 2-Methylpentane Cyclopentane
B.P., 27 9 36 1 49 7 58 0 60 3 49 2
C.
Table I shows a smooth relationship between boiling points and retention time within a homologous series. I t also shows that the order of arrival does not follow increasing boiling points when different classes of compounds are analyzed by this column. This difference makes possible the separation of close boiling components such as 2,4-dimethylpentane and cyclohexane. However, this column mill not separate compounds with the same retention times such as cyclopentane and 3-methylpentane. The naphthenes and aromatics are more soluble in the diortyl phthalate than the paraffins n-ith comparable boiling points; thus their retention times are longer. The versatility of this method can be greatly increased by varying the properties of the liquid phase. A column in which the granular coating was a paraffin wax was used for separating cyclopentane and 3-methylpentane, which could not be separated a i t h the dioctyl phthalate column, A vacuum pump oil (Octoil-S, Consolidated Vacuum Corp., Rochester, K.Y.) was found to be an effective coating for separating several hydrocarbons that could not be separated on the dioctyl phthalate column. This is illustrated in Figure 3 shon-ing the separation of a small amount of trans-2-pentene from 2-methvl-2-butene. This sample was
Order of Elution for Several Hydrocarbons Retention Time, Nin.
10 12 17 22 22 24
Component 3-Rfethylpentane n-Hexane 2,4-Dimethylpentane Cyclohexane n-Heptane Benzene
B.P., C. 63.2 68.7 80.6 80.8 98.4 80.1
Retention Time, &fin. 24 28 36 53 67 87
ANALYTICAL CHEMISTRY
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Table 11. Analysis of Sjnthetic Cj and Cg Hydrocarbon Mixture, Mole $% Observed, % Component Isopentane n-Pentane 2,Z-Dimethylbutane 2,3-Dimethylbutane 2-Methypentane 3-Methylpentane n-Hexane Av. % diff.
Blended, % 6.6 16.7 5.8
Run 1 6.7 16.0 5.9
5.9 5.9 29 8 29.3
11.8
0 1 ,
70
, , , , , , 65
Figure 5.
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50
I ' 45 40 -MINUTES
I " " " '
35
I "
30
+o
25
I
20
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I ' I5
'7 0
Chromatogram of synthetic mixture of CS and Cg hydrocarbons
Figure 6 is the chromatogram of a more complex mixture of CS through C7 hydrocarbons. This sample was run in the same column as in Figure 4 a t a temperature of 85" C. and a gas flow rate of 29 ml. per minute. This chromatogram shows several cases wherein two components have similar retention time; thus they come off together as a single peak. However, this curve shows the difference in retention time for benzene and cyclo compounds compared t o the paraffins of similar boiling points. Development work is now concentrated on the application of this technique as a separation tool to use in conjunction with the
mx
Run 2 6.7 17.3 5.8 11.2
1
-0 7 10.1 0.0
-0 4 +0.9 0.4
29.4 30 2
run in a column 14 feet long with an inside diameter of 4.5 nim. operated a t 65" C. with a carrier gas (hydrogen) flow rate of 18 ml. per minute. For the chromatogram (A) using dioctyl phthalate as the liquid phase, incomplete separation was obtained. Holvever, for the chromatogram (B) using Octoil-S a t the same conditions, complete resolution of the trans-2-pentene and 2methyl-2-butene was possible. Using a larger sample charge and calibrated peak areas for the minor components, this technique is useful to determine small percentages of impurities in pure grade hydrocarbons. Figure 4 is the chromatogram of a sample which is typical of many encountered in hydrocarbon research. This sample was run in a column 14 feet long with an inside diameter of 4.5 mm. operated a t 65' C., and a carrier gas (helium) flow rate of 18 ml. per minute, using Octoil-S as the liquid phase. The different components in the mixture are marked by the individual peaks. This chromatogram shows complete separation of all components in this particular mixture. Figure 5 is the chromatogram of a synthetic mixture of C5 and Cg hydrocarbons. The same column and conditions were used for this sample as in Figure 4. This mixture is tvpical of many samples now analyzed by this technique for control purposes. As 2,3-dimethylbutane and 2methylpentane have the same retention time on this column, they could not he separated. Table I1 is a comparison of the blended percentages and observed percentages for the synthetic mixture shown in Figure 5. The average deviation in terms of total sample for run 1 is 0.4% and run 2 is 0.6%.
Diff.
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+0.1 +0.6 0.0 -0.6
-1 2 +I I 0.6
28.6 30 4
mass spectrometer. This technique would eliminate the timeconsuming precision distillation now in use to separate these cuts. A charge of approximately 0.02 to 0.03 ml. of a light gasoline is sufficient to provide several cuts in the C5 to C, hydrocarbon range for further analysis by the mass spectrometer. Table I11 shows a comparison between the blended concentration and the observed concentration for the 18-component misture shown in Figure 6. Although some peak areas are in error by more than 170, the average error in terms of total sample is only 0.6%.
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72
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TIME IN
Figure 6 .
24
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8
MINUTES
Chroniatogram of an 18-component hydrocarbon mixture
.4n example of the technique applied to a veiy difficult separation is shown in Figure 7 , which is the chromatogram for a complex heptene mixture. This sample was run in a column 14 feet long with an inside diameter of 4.5 mm. operated at 65' to 85' C. with a carrier gas (hydrogen) flow rate of 29 ml. per minute, using dioctyl phthalate as the liquid phase. By inspection of this chromatogram, it is obvious that the mivture contained several compounds as indicated by the breadth of some of the peaks and the fact that the curve did not return to the base line betmeu peaks. The tentative identification of the peaks is made by correlating boiling points and retention times for several of the pure compounds that are available for calibration purposes. Since all of these components were not used in the calibration, this is onlv a tentative identification of the peak areas. Figure 8 is a photograph of the apparatus used in the gas analysis laboratory for the routine analysis of liquid samples. This unit is assembled on a metal frame which is mounted on a dolly. The chromatographic column, which is assembled in the glass xater jacket, is mounted on the left side. The insulation from this jacket is removed so the arrangement of the column would he visible for this picture. Water from the constant temperature bath is continuously circulated through the jacket to maintain the desired column temperature. The flow of carrier gas is indicated by the rotameter on the side of the apparatus and is measured by the wet test meter located beneath the constant temperature water bath. The thermal conductivity cell is enclosed in a constant-temperature air bath located directly
t
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
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Table 111. Comparison between Blended and Observed Concentrations for 18-Component Blend component 1aopentane n-pentane 2.2-Dimethylbutane %Methylpentme 2.3-Dimethylbutane 3-Methylpentane cyo1opentane n-HeX&ne 2.4-Dirnethyl~entane 2.2-Dimethylpents,ne
Figure 7.
Biended %
Observed %
4.5
4.7
5.8
5.8
6.3 5.4
11.8
3'%
% Difference
CompOnent
+0.2 +0.5
2,2.3-Trimethylbutane Methyloyolopentane
5.6
-0.3
-1.6
3.3-Dimethylpentane 1.1-Dimethylpentane Cyclohexane
5.9
10.2
10.8
11.5
+0.7
10.6 6.9
10.9 6.4
+0.3 +0.5
"-Heptnlle BenZene Methyloyolohexane
5.9 5.9 5.9
6.7
+0.8 f0.9
11.8
10.1
-1.7
-0.4
Chromatograin of a complex heptene sample
Blended
9.4
Observed %
% DiKerenoe
9.5
6.8
5.9
AT. % diff.
f0.1
0.0 0.6
under the recorder. This bath is equipped with a mercury type thermoregulator, electronic relay, circulating fan, and heater for maintaining the thermal Conductivity cell a t a constant temperature. A modified Brown electronic recorder having a 0-to 5mv. range and a chart travel speed of 1.3 om. per minute is used to meamre the signal from the thermal conductivity cell. This instrument range and chart travel speed provide sufficient area under the peaks for accurate measurement with a planimeter. This unit is operated by the high schoolgraduate technicians and used far operating the low temperature distillation apparatus. One of the features of this model is the ease and simplicity of operation. As soon a8 the sample is charged to the column, the apparatus is completely automatic. Less than 5 minutes of operator time is required to charge the sample and 10 to 20 minutes for calculating the peak areas. As soon as the last component in one sample is eluted, the column is immediately ready to be charged ~ t another h sample. Several hundred routine samples have been analyzed in the same dioctyl phthalate column with no apparent change in accuracy or reproducibility. I n conclusion the scope and accuracy obtained by this simple procedure should be emphasized. This technique is certain to supplement and in some cases supplant the use of infrared spectrometers, mass spectrometers, and distillation apparatus for the analysis of complex organic mixtures. ACKNOWLEDGMENT
The author8 wish to thank N. D. Coggeshnll for his interest and help in carrying out this work, and J. R. Tomlinson, Lionel Domash, and G. F. Crable for their help in the initial stages of this project. The authors are also indebted to the management of Gulf Research & Development Co. for permission to publish this paper. LITERATURE CITED
(1) Bradford, B. W., Harvey. D., and Chalkey. D. E., J.,lnat. Petrolmm, 41, 80 (1955). (2) Burrell Corp., Pittsburgh, Pa., "The Fracton," Booklet 82,
1954. (3) Griffiths,J., James, D., and Phillips. C., Analyst, 77,897(1952). (4) James, A.,Biochem. J. (London). 52,242 (1952). (5) James, A. T.,Mfg. Chemist, 26, 5 (1955). (6) James, A. T.,Research (London), 8,8 (1955). (7) James, A,, and Martin, A.. Biochem. J . (London), 50,679 (1952). (8) James, A. T.,and Martin, A. J. P.. Bdt. Med. Bull., 10, 170
(1954). (9) James, A., Martin, A,, and Smith, G., B i o c h a . J. (London).52, 238 (1952). (IO) James. D., and Phillips, C., J . Chem. Soe., 1600 (1953). (11) Patton, H.,Lewis, J., and Kaye, W., ANAL. CHEM.,27, 170 (1955). (12) Phillips, C..Dismsiona Faradau Soe., 7, 241 (1949) (13) Ray, N.,J. A p p l . Chem. (London), 4,21 (1954). (14) Ibid., p. 82.
F i g u r e 8.
P h o t o g r a p h of r o u t i n e apparatus
RECE~VED for reriew April 8, 1955. Accented dune 30, 1955.
